Put the helper function to anywhere. Copy, Paste, Include or autoload checkExtension() function.
How tom use?
checkExtension('Ebter the extension you want to check', 'Enter media type: video, image, audio')
Example:
var_dump(checkExtension('png', 'image'));
It will return 'true' if the extension were found in the array and 'false' if it were not found.